Engine losing power, shifting wrong, coils already replaced. What next?
 

I've searched the forum, but the only threads coming up related to loss of engine power are recommendations to replace the coils. I replaced the coils in my wife's 8 back in July. It's been running fine since then, but today it started to do the same thing where after the engine is warm, and you push on the gas from a stop (light or sign), the engine chugs lower and lower like it's about to stall. Also, the transmission is shifting wrong (It's my wife's car, so don't rip me for an auto transmission in this car). I have not yet tried taking the car out and using the paddle shifters or stick manually to shift.

Vitals: 2005 series I, had the engine recall work done, 40K miles, I replaced coils, plugs and wires in July (so about 2.5 months ago). No other maintenance issues.

Any advice on how to isolate the problem or problems?

Things to check on:
- Clean MAF
- Verify no vacuum leaks
- Test compression
- Check front O2 sensor response
- Check air filter for clogging
- Check for oil in the intake
- Check for intake screens being in place and straight
- Transmission fluid flush within specified interval
- Battery cable terminals and clamps clean and secure (remove, clean, and re-secure, they can corrode free and still apparently still be clean and secure, have to remove to verify)
- Engine block ground clean and secure
- Transmission grounds clean and secure
- Chassis grounds clean and secure


um, think that is it for the moment.

Still tons of things it could be.

For clarity, the stumbling indicates a difficulty igniting the air/fuel mixture properly. At low rpm, the engine is more sensitive to air flow variances (1 gram per second is a 20% swing at idle, only a 0.5% swing at full throttle 8,500rpm) and ignition variances (lower compression at low engine speed and more time for the mixture to condense on the walls makes ignition a bit harder).

Each of the above points I listed can play with one or the other, or both, and will be more prominent at low engine speed typically.

Many of these also have problems at high RPM, though in a different way, since high RPM issues typically relate with ignition or fuel trying to keep up with how fast things are happening, a general increase in heat of components, and/or a capping restriction on air or fuel flow.


The automatic transmission controller uses throttle request, RPM, and engine load to determine what gear to shift into, and engine load is calculated based on RPM, MAF, and... (i forget the 3rd?), so if you have something throwing off the ignition, air, or fuel, then your load value can drop to where the transmission controller doesn't see that it should be shifting.

If you try driving with the manual mode and it still has trouble shifting, then look at the transmission controller related items, like the ground, the fluid, etc... If manual mode clears up the shifting problem, then it points the problem back to the health of the engine, it's airflow, and it's spark.

I've done the basic maintenance items, and the engine sluggish/stall still intermittantly occurs. I guess the next step is to take it in somewhere for a compression test. I'm also wondering if the replacement coils I purchased were lemons, there were generic aftermarket parts "sourced from the OEM manufacturer".